It is known that vertical take-off aircraft, and in particular helicopters, need to have shock absorbers that are capable not only of absorbing shock during normal landing, but also of absorbing energy in the event of a crash landing, with the energy-absorption function being intended to avoid, if possible, damage to the aircraft itself, and in any event to reduce the stresses suffered by the bodies of passengers in the aircraft. It is also often required that the shock absorber should be raisable so as to enable the landing gear to be retracted into an appropriate housing in flight.